In order to extend the shelf life of a food product, bacteria within the food must be eliminated. One common method for eliminating harmful bacteria in food products is by heating the food products to a temperature sufficient to kill the bacteria. For many years, food products were first sealed within metal cans and then the canned food was heated to a suitable temperature. After cooling, the canned food could be stored at room temperature for long periods of time.
Recently, metal cans have been replaced by flexible pouches called retort pouches. These pouches come in a variety of shapes and sizes. The two most common forms are described as flat or pillow shaped and gusset or stand-up shaped pouches. These pouches are considerably lighter and lower cost than metal cans, and are considered as a source of reduction when placed into the public waste stream. The typical flat or pillow pouch consists of one or two sheets of laminated material. The typical gusset or stand-up pouch is manufactured by using three sheets of laminated material. Both types are sealed together by heat-sealing after filling. This process can be accomplished by using pre-made pouches, filling and sealing out of line, or by form, fill, and seal inline.
In order to be used in a retort process, the flexible pouch must comply with a number of requirements. The U.S. government Food and Drug Administration (FDA) is very specific as to the materials which can and cannot be used for flexible packages that will be subjected to temperatures over 250° F. Specifically, 21 C.F.R. § 177.1390 regulates the chemical components that can be used to construct a flexible pouch that will be subjected to these extreme temperature environments. The physical properties of these flexible pouches: lamination bond strengths, heat seal strengths, WVTR (water vapor transmission rates), OTR (oxygen transmission rates), and burst analysis exceed normal testing parameters, but the liquid components used to adhere the films together, along with the films themselves, must comply with migratory testing guidelines established by the FDA.
The components that make up the pouch (ink, adhesive, films, and solvents) must not contain mobile components that might contaminate the contents, and include a functional barrier that prevents the passage of mobile substances (gases or volatile liquids) from outside the pouch. Any liquid, that is to say, in this case the adhesive, that is used inside of the functional barrier must be subjected to migration testing if the chemical nature of that adhesive is not made from an aliphatic isocyanate. The regulation is waived if the adhesive components are of an aliphatic nature. In such the adhesive is deemed FDA § 177.1390 compliant. The pouch still must remain airtight and bacteria-proof (hermetically sealed) after going through the normal retort process, which exposes the pouch to temperatures in the range of from 120° C. to 130° C. for 30 to 80 minutes at a pressure of 3.0 to 5.0 Bar. This temperature, pressure, and time may be varied slightly depending on the pouch size and the contents within the pouch.
One type of flexible retort pouch that is currently available is constructed of a four layer laminate with an outer layer of polyethylene terephthalate (PET), a metal foil layer, a polyamide layer, followed by an inner layer of cast polypropylene (CPP). The layers of such prior art packages are held together by urethane adhesives that contain methyl ethyl ketone, ethyl acetate, or acetone as solvents. Until recently, solvent based adhesives were the only type that could be used to manufacture retortable structures.
Solventless adhesives for retort applications have been developed only in the last two years. These solventless adhesives are defined as a two-component urethane system in which one component is isocyanate and the other component is polyol. The isocyanate part is aliphatic to meet the FDA guidelines for retortable materials. These components are low molecular weight in nature, and require no solvent for dispersion or application. When the two components are combined in the proper mix ratio, and applied on a solventless laminator at the proper coating weight and temperature, and with the proper cure time, the end result is a crosslinked polyester urethane film that adheres the adjacent layers together within a retort lamination.
The previously-used laminate described above requires four distinct layer materials, three adhesive coatings, and three laminating operations, requiring either complex and expensive multi-layer laminating machine or a complex succession of operations. As a result, the cost of manufacturing such a material is significant. However, in a multi-layer laminate, each layer has a definite purpose. For example, PET provides an abrasion-resistant, printable outer surface. Nylon provides structural strength to the film. Metal foil provides impermeability to gases and, in particular, prevents oxygen from penetrating the contents of the package and thus increases the shelf life of many products. Cast polypropylene provides a heat-sealable inner surface that can be welded to itself to form an airtight package, with a softening temperature low enough to be sealed without damaging the contents, or the laminate structure, but high enough to survive the retorting of the sealed package. Thus, the laminate structure cannot be simplified merely by eliminating layers.